While our friends inthe GLOC were playing at Road America this weekend (and having more fun than me, I'm sure...) I spent saturday balancing parts in my 5.0 liter sleeved 928 motor.

Was surprised to find out how far apart the OEM connecting rods were - up to 3.1 grams difference between them at the worst. These were all out of the same 310 HP "S" motor....

We checked them for twist and bends, then polished off all the stress risers, then balanced them to within 1/10th of 1 gram to each other. Normal engine balancing calls for balancing the components to within 1 gram, but it is pretty easy to balance them to 1/10th of a gram, so we did.

The Pistons were not too much different - factory new ARIAS pistons that varied in weight by more than 2 grams also. Got them all to witihn a 1/10th of each other.

Now the whole rotating assemply is at the machine show being dynamically balanced to match the counterweights on the crank...

Photos follow.

 

Why didn't they nip material out under the wrist pins on the pistons instead of milling the underside of them? I had the GM balanced out and I was amazed at how much they took off the rods, but she is smooth now.

Carl, I am curious as to why you choose the sleeves and not the plating process?

 

Yes, More info on the sleeves Carl.

Also, Arias? How where they on design help, and implementation of your ideas on compression and ring land height, etc.

 

The pistons are balanced by removing material from non-critical areas of the piston wrist pin bosses. In this case, from the sharp angle created by the end of the forging, by radiusing it out. This gave us the neccesary material to work with and also allowed us to remove a sharp corner and a stress-point in the same action.

This block was already sleeved when I bought it - but I prefer that for this application rather than nikasil plating - as this is/will be a blown engine, we have hopes that the steel sleeve will help us resist trumpeting of the cylinder walls under pressure. The 928 is an unsupported wet-wall design, and under enough boost, some trumpeting of the cylinder walls can occur causing casting fractures.

Not at the 8 psi kit SC level, mind you - but in case I "go nuts" and reach for double-digit boost numbers, I thought the steel wall would be nice to have.

 

Brendan C - They were fine, but I went thru Phil Threshie at 928 Developments on purpose because I figured he has ordered more 928 pistons than I have and I wanted the benefit of his expertise.

We did opt up for the special High Silicone alloy (4042, I think...) - that's a special order item - but the high-silicone content reduces piston expansion quite a bit. That allows you to hold the bores to a tighter tolerance....

On the norm, you might hone the bore to within .008" of the piston size to allow for expansion of the piston. With the high-silicone alloy piston, you are able to hone to .0035" of the piston (that's what we did) - and that means much less blow-by.... what I was trying to reduce in a supercharged motor.

 

How much horsepower does balancing a blown 5 liter add?

Looks like nice work! How much would this service cost?

 

Let me anticipate the answer. Balancing doesn't add horsepower, it allows more horsepower to be produced safely.

 

I would think that a balanced engine should be able to rev faster though with everything running in perfect balance, which would knock off time to an given acceleration mark. Has anyone actually checked to see the difference in performance of a balance shark engine running boost around 500 chp? I doubt it.
It would like having a crappy set of wheel bearings vs. a nice set of polished ones with good lube, one would spin more freely and therefore give that wheel an edge on speed.

Just my hypothesis.

 

How was the steel sleeve put into the block? Wet or dry sleeve? Pics?

Was the sleeve T'd at the top?

 

The sleeve was pressed in using very conventional methods.

The bore is bored out oversized almost - but not quite - all the way down. Leaving a little "shelf" of aluminum at the bottom of the bore. Then the dry steel alloy sleeve is pressed in all the way down to that shelf. It is an interference fit.

The little shelf at the bottom helps clamp the sleeve in place between the bottom and the head when it is torqued in place, so it cannot move around.

 

Warren928 - Balancing a motor pays dividends in longevity, more than HP directly. The cost os not too much - many shops will balance rods and pistons to within a gram for about $200, then, after that is done, you want to spin-balance the entire rotating assembly (crankshaft) on a special machine they have. My engine machine shop is charging me $120 to do mine.

In the scheme of things - the cost of a complete engine rebuild - a bit of engine balancing is some cheap insurance that it is going to be smooth and not shake itself apart.

The time-consuming part is the static balance of the individual rods and pistons - and I wanted to go to .1 gram instead of 1.0 gram - and I have that equipment in my shop - so I did that myself.

In this case - balancing the engine was a more of a "need" than a "want" because the crankshaft has counterweights on it designed for a cast 4.7 liter Mahle piston up top, and now it is going to have forged 5.01 pistons up top - you just know they do not weigh the same, so some weight will have to be taken out of the crankshaft counterweights to rebalance this assembly.

 

Very cool Carl! The notion of a sleeved engine appeals to me. I'd like forced induction but in my case, with 10.4:1 compression, it's either low boost or rebuild the motor with low compression.

Sleeving! What shop did this for you- and how much did it cost? How much did you pay for the pistons and all the machine work?

N!

[Boost rules the roost but there's no replacement for displacement~]

 

I have a question on dynamic balancing. I know they clamp weights onto the rod journals and spin the crank but I always wondered how they determine that weight?

I would think that if they use the entire weight of the piston/rod/pin/bearing, then the crank could be balanced on a machine becuase the clamped weights spin with the crank but, on the engine, the reciprocating weight and how it interacts with the crank changes (becuase the piston and top part of the rod don't spin around with the crank). I would think the assembly would only be in balanace at TDC & BDC.... at any angle other than those 2 points, the cranks counter weight would be acting against the lower weight of the large rod end and bearing, thus the assmebly would be unbalanced for much of 1 full rotation?

Can anyone describe how that works?

 

Why not remove the square balance pads totally? Or almost. Also you have the weight of the rods at about 590 grams, I thought I got 850 grams when I weighed my 5 litre rods. Those were the last of the sinter forged ones. Am I right or were my kitchen scales off?

Thanks

 

There are two ways that the rotating assembly of a piston engine are balanced - called an internal balance or an external balance.

Internal is preferred, but costs more money, externally balanced motors are cheaper to build and common on hi-volume production cars.

An externally balanced motor will show weights welded onto the flywheel to make up for the out-of-balance condition of the motor. This means that the flywheel is now POSITIONAL - and must always follow that crank and also only be installed one way each time.

An internally balanced motor is balanced without the flywheel or harmonic balancer mounted. The flywheel and harmonic dampener are balanced to "neutral" separately so it does not matter in what position they are mounted to the motor later.

They calculate the "flyweight" by just adding up the weight of the piston, rings, wrist pin, wrist pin keepers, rod, rod bearings, and rod nuts. This is the total flyweight for a single cylinder. They attach that weight to each rod journal and spin the crank - drilling out or welding to the crank weights as needed.

Keep this in mind: an old adage (from an old mechanic) about wheel alignment and balancing: that a tire/wheel combo that is 1 ounce out of balance will hit the pavement with 10 pounds of force at 60 MPH. Why you balance your tires.
Imagine a piston/rod assembly reciprocating at 3200 feet per second!